Interior and exterior construction boards with cores of plaster, cement, or hybrid materials, such as cement boards or gypsum boards, are used in a wide variety of indoor and outdoor structural applications. For example, cement boards are used as a support surface for overlying materials such as wood siding, stucco, synthetic stucco, aluminum, brick, tile, stone aggregate and marble. In addition, cement boards are used in exterior insulating systems, commercial roof deck systems, and exterior curtain walls.
Generally, cement boards contain a core formed of a cementitious material which is interposed between two layers of facing material. The facing material advantageously contributes flexural and impact strength to the high compressive strength but brittle material forming the hardened cementitious core. In addition, the facing material can provide a durable surface and/or other desirable surface properties to the cement board.
One material which has been used to form facing sheet material is glass fibers. In addition to increased dimensional stability in the presence of moisture, glass fiber facings provide greater physical and mechanical properties to the cement board. As described, for example, in U.S. Pat. No. 5,371,989 to Lehnert et al., the method of making cement boards containing glass fiber facing sheets typically consists of providing a continuous feed of the facing material and depositing a cementitious slurry onto the top surface of the facing material. A second continuous feed of facing material is then applied to the top surface of the slurry. The slurry is then dried to harden the cementitious composition and to integrate the facing material in the cement board. The cement board is cut to a predetermined length for shipping and for its eventual use.
U.S. Pat. No. 4,647,496 to Lehnert et al. describes the use of randomly oriented fibrous glass mats as the facing material for gypsum boards. Additionally, U.S. Pat. No. 5,397,631 to Green et al. describes coating a gypsum board including a randomly oriented glass mat with a water-resistant resinous binder to prevent the deterioration of the bond between the gypsum and the glass mat.
An alternative to the randomly oriented fibrous glass mats to reinforce cement boards is the use of open mesh glass scrims formed from continuous glass yarns. Because these scrims contain continuous yarns, they possess greater tensile strength than the randomly oriented fibrous glass mats. For example, U.S. Pat. No. 3,993,882 to Knauf et al. describes using a facing sheet formed of a glass fabric. In addition, published European patent application 637,658 to Bay Mills Limited describes the use of glass rovings to form nonwoven scrims for stucco boards.
In order to reduce glass usage, the mesh size of the glass scrims may be increased thus reducing the number of yarns or "picks per inch" in the transverse and/or the longitudinal direction without reducing the strength of the cement board. Nevertheless, the resulting increase in the size of the mesh openings in the glass fiber facing sheet causes other problems. Particularly, when the open mesh facing is applied to a cementitious slurry, pitting or the formation of indentations may occur in the center of the mesh openings which negatively affects the aesthetic appearance and surface properties of the cement board.